A number of different types of apparatus have been proposed for recycling asphaltic concrete, including those disclosed in U.S. Pat. Nos. 3,674,242, 4,130,364, and 4,075,710. Such apparatus, although useful in heating used asphaltic concrete particles, have a common drawback of being quite inefficient in heating the particles to useable product temperatures, normally above about 225.degree. F., and preferably above about 275.degree. F. A major problem with such apparatus in adequately heating the particles, a substantial amount of which are quite small and contain large amounts of asphalt, is that unless heated gradually, and without being directly exposed to the hot gases of combustion below about 1,000.degree. F., significant quantities of volatile asphalt materials are given off and are vented directly to the atmosphere in the form of smoke, noxious fumes and the like. Venting of such unburned hydrocarbon volatiles and smoke from the hot asphalt is contrary to acceptable environmental pollution standards, especially in or near urban areas. Because of such prior apparatus limitations, 100% asphaltic concrete recycle processes are not used, and instead, substantial amounts of aggregate are introduced and heated at the hot drum end, prior to introducing the recycle material downstream in a cooler drum portion.
In other U.S. Pat. Nos. 3,674,242, and 4,130,364 asphalt-aggregate composition is introduced into one end of a rotating drum and gradually heated as it is advanced toward a burner at the opposite drum end. However, such systems are also relatively inefficient because the hot exhaust gas, which includes any volatile asphaltic hydrocarbons, is simply vented directly to atmosphere from the drum end opposite the burner. Accordingly, these systems offer no advantage over those of the previously cited patents, albeit the composition flows counter to the hot gas flow, since the gas passes directly through the length of the drum, into one end and out the other.
In my prior co-pending applications Ser. Nos. 871,351, filed Jan. 23, 1978 now U.S. Pat. No. 4,208,131, and 906,734, filed May 17, 1978 now U.S. Pat. No. 4,240,754, there are disclosed apparatus for heating asphaltic concrete particles by direct exposure to hot gases of combustion, but in which hydrocarbon volatiles and smoke are burned prior to being vented into the atmosphere. Such apparatus is primarily intended for heating limited amounts of material. It is to an adoption of my prior apparatus which would be useful for recycling material from an entire lane or lanes of highways or roads that the present invention is directed.